1. Field of the Invention
The present invention relates to a fuel supply system for an internal combustion engine, and more particularly to a fuel supply system for an internal combustion engine having first fuel injection means (intake manifold injector) for injecting a fuel into an intake manifold and/or an intake port, and second fuel injection means (in-cylinder injector) for injecting a fuel into a cylinder.
2. Description of the Background Art
An internal combustion engine provided with an in-cylinder injector (fuel injection valve) for injecting a fuel into a cylinder and an intake manifold injector (fuel injection valve) for injecting a fuel into an intake port, and controlling the in-cylinder injector and the intake manifold injector in accordance with an operation state to inject the fuel by combination of intake manifold injection and in-cylinder direct injection is known (e.g., Japanese Patent Laying-Open No. 07-103048).
In a fuel supply system for the internal combustion engine, generally, one fuel line extending from a fuel tank toward the internal combustion engine is branched in the vicinity of the internal combustion engine so as to supply the fuel to an intake manifold injector and to an in-cylinder injector. With this configuration, however, the fuel line has a complicated configuration in the vicinity of the internal combustion engine, and the fuel supplied from the fuel tank may be subjected to a great amount of heat from the engine block of the internal combustion engine. In general, the fuel supplied to the intake manifold injector is a fuel of a low pressure that is pumped up by using a low-pressure fuel pump. As such, it has been pointed out that the fuel, when subjected to the great amount of heat from the engine block, may partially vaporize in the fuel line or a delivery pipe for supplying the fuel into the intake manifold injector, leading to occurrence of vapor lock.
To address such a problem, for example, Japanese Patent Laying-Open No. 2004-278347 discloses a fuel supply system in which a fuel tank, a low-pressure fuel pump, a fuel pressure regulator (pressure regulator), an intake manifold injection (low-pressure) delivery pipe, a high-pressure fuel pump, an in-cylinder injection (high-pressure) delivery pipe, and a relief valve are arranged in series. In this fuel supply system, it is possible to prevent fuel injection failure attributable to the vapor lock caused in the pipe connected to the intake manifold injector with a simple configuration.
In the fuel supply system for an internal combustion engine as described above, it is also necessary to take account of various kinds of problems, besides the occurrence of the vapor lock, in association with the fuel supply to a plurality of systems.
Firstly, it is necessary to achieve a configuration that can improve oil tightness of the injectors during stop of operation of the vehicle, so as to maintain good exhaust property at the time of next start of the engine.
In the fuel supply system disclosed in Japanese Patent Laying-Open No. 2004-278347, for example, the intake manifold injection (low-pressure) delivery pipe is arranged downstream of the fuel pressure regulator. Thus, even if an electromagnetic relief valve for releasing pressure is arranged downstream of the in-cylinder injection (high-pressure) delivery pipe, it is difficult to intentionally release the fuel pressure of the low-pressure delivery pipe at the time of stop of operation of the vehicle. This leads to poor oil tightness, and there may occur leakage of the fuel from the intake manifold injector during stop of operation. Such leakage of the fuel may lead to degradation in exhaust emission property at the time of next start of the engine.
Further, it is necessary to achieve a configuration that enables preferable fuel injection in accordance with a temperature of the engine.
For example, in a cold state of the engine, atomization of the fuel within the cylinder would not be promoted, so that the fuel injected into the cylinder tends to adhere to the top face of the engine piston (piston top face) or to the inner peripheral surface of the cylinder (cylinder inner face (bore)) in a large amount. Of the fuel thus adhered, especially the fuel adhered to the piston top face will be gradually atomized during the subsequent engine combustion process, and discharged from the cylinder in the state of imperfect combustion. This will cause generation of black smoke, increase of unburned components and the like, leading to degradation of exhaust emission property. Further, the fuel adhered to the cylinder inner face will be mixed with the lubricant applied to the cylinder inner face for lubrication of the engine piston. This will cause dilution of the lubricant by the fuel, i.e., so-called fuel dilution, so that the lubrication property of the internal combustion engine may be impaired.
Thus, taking account of the adverse effects of degraded exhaust emission property as well as lowered lubrication property of the internal combustion engine, it is preferable to inject the fuel from the intake manifold injector at the time of homogenous combustion operation in the cold state of the engine, while avoiding the fuel injection from the in-cylinder injector.
In the fuel supply system disclosed in Japanese Patent Laying-Open No. 2004-278347, however, the intake manifold injection delivery pipe and the in-cylinder injection delivery pipe are connected in series, with the intake manifold injection delivery pipe located upstream of the in-cylinder injection delivery pipe. As such, in the cold state of the engine, the temperature increase of the fuel within the in-cylinder injection delivery pipe is slow, and the atomized particle size of the fuel injected from the intake manifold injector tends to become large, leading to degradation of exhaust emission property.
Meanwhile, when the operation of the internal combustion engine proceeds (i.e., in the warm state of the engine), the temperature at the tip end of the in-cylinder injector increases with the fuel combustion within the combustion chamber, and deposits tend to be produced in the injection hole at the tip of the in-cylinder injector, which cools the in-cylinder injector. Therefore, it is preferable that the fuel temperature within the in-cylinder injection delivery pipe is low.
With the configuration disclosed in Japanese Patent Laying-Open No. 2004-278347, however, the in-cylinder injection delivery pipe is arranged downstream. Thus, only the fuel of the quantity corresponding to the overall fuel quantity supplied from the fuel pump from which the fuel quantity injected from the intake manifold injector is subtracted, is applied with pressure by using the high-pressure fuel pump and provided to the in-cylinder injection delivery pipe. That is, since the fuel flow rate within the in-cylinder injection delivery pipe is small, the fuel temperature within the in-cylinder injection delivery pipe tends to increase due to the heat received from the internal combustion engine. As such, the effect of cooling the in-cylinder injector is not obtained sufficiently, which is disadvantageous in view of accumulation of deposits.
Still further, since there exist a plurality of fuel systems (of high pressure and of low pressure), it is also necessary to achieve a configuration that can prevent vapor lock within the fuel supply system attributable to the difference in fuel injection quantity between the fuel systems.